1. Field of the Invention
The present invention relates to a nonaqueous polymer cell, and it particularly relates to a nonaqueous lithium cell having a lithium ion conductive polymer.
2. Description of the Related Art
Accompanying with the recent innovation in electronic appliances, the advent of a new high-performance cell is expected. The batteries used as electric power sources in present-day electronic appliances mainly are primary batteries such as manganese dioxide-zinc cells and secondary batteries such as lead acid cells and alkaline cells, e.g., nickel-cadmium cells, nickel-zinc cells, and nickel metal hydride cells.
As the electrolyte of these batteries, an alkaline solution, e.g., potassium hydroxide, or water sulfuric acid solution or the like is used. The theoretical water decomposition voltage is 1.23 V. If a battery system is designed so as to have a potential higher than that value, water decomposition is to occur and stable storage of electrical energy is difficult. Accordingly, batteries of the above kinds which have been put to practical use have an electromotive force about 2 V at the most. Consequently, 3-V and higher-voltage batteries should employ a nonaqueous electrolyte. A representative example thereof is the so-called lithium cells using a negative electrode containing metallic lithium.
For example, as lithium primary batteries, there are manganese dioxide-lithium cells and carbon fluoride-lithium cells, while as lithium secondary batteries, there are manganese dioxide-lithium cells and vanadium oxide-lithium cells.
The second batteries having the negative electrode containing lithium metal are apt to occur a short circuit due to the dendritic deposition of metallic lithium metal, so that their cycle life is short. In addition, because of the high reactivity of metallic lithium, it is difficult to ensure safety. For these reasons, the so-called lithium ion cell has been commercialized which employs graphite or carbon in place of lithium metal and employs lithium cobaltate or lithium nickelate as the positive active material. There recently is a requirement for a highly safe cell having higher performance with increasing demand as high energy-density cells.
In lithium cells and lithium ion cells (hereinafter collectively referred to as lithium cells), most of the lithium ion participating in electrode reactions during charge and discharge are not the lithium ion which is originally dissolved in the electrolyte but the lithium ion which is released from the active material of an electrode and reaches the opposite electrode through the electrolyte. The lithium ion hence moves a long distance. In addition, the transport number at room temperature of lithium ion in the electrolyte in lithium cell is usually 0.5 or lower, while the transport number of proton and hydroxide ion in aqueous-solution-electrolyte cells is nearly unit. The moving speed of lithium ion in an electrolyte strongly depend on the diffusion of the ion. Since organic electrolytes have a higher viscosity than aqueous solutions, the diffusion of ion is slow. Therefore, the lithium cells have a problem that they are inferior in high-rate charge/discharge performance to cells employing an aqueous electrolyte.
In the lithium cells described above, a microporous film made of, e.g., polyethylene or polypropylene is used as a separator. For producing such microporous films, a casting-extraction process and a stretching process are mainly used. The wet process is a process for producing a non-directional microporous polymer film in which a polymer is dissolved into a liquid, the solution is spread into a sheet, and the sheet is immersed in a bath to remove the liquid serving as a solvent for the polymer and to thereby form pores (U.S. Pat. No. 4,539,256). This microporous film separator having circular or elliptic pores is used in closed nickel-cadmium cells (U.S. Pat. No. 5,069,990). In the stretching process, a microporous film is produced by stretching a polymer film to form directional pores therein (U.S. Pat. No. 4,346,142), and this microporous film is extensively used in secondary batteries. In another process, fine particles of salt, starch, or the like is added to a polymer, the mixture is formed into a sheet, and then the fine particles are dissolved into a liquid to remove the same to thereby produce a microporous polymer film (U.S. Pat. Nos. 3,214,501 and 3,640,829). Another process for producing a microporous polymer film comprises dissolving a polymer into a liquid at a high temperature, cooling the solution to solidify the polymer, and then removing the solvent (U.S. Pat. Nos. 4,247,498 and 4,539,256). By utilizing a shutdown effect in which the pores of the microporous polymer film are closed upon thermal fusion of the film, a separator is served as a safety device for the cell. (J. Electrochem. Soc. 140(1993)L51). Even if this cell comes into a dangerous state as a result of heat generation caused by internal short circuit in the cell, the safety device functions to insulate the positive electrode from the negative to thereby inhibit further reactions at the positive and negative electrodes.
Lithium cells have a problem concerning their safety because of the use of a flammable organic electrolyte as the electrolyte, unlike the batteries employing an aqueous electrolyte, such as lead storage batteries, nickel-cadmium cells, and nickel metal hydride cells. Accordingly, a solid polymer electrolyte having lower chemical reactivity is used in place of the organic electrolyte so as to attempt to improve the safety thereof. (Electrochimica Acta, 40(1995)2117). Use of solid polymer electrolytes is being attempted also for the purposes of producing a flexible cell, simplification of cell fabricating steps, reduction of production cost, etc.
The ion conductive polymers which have been investigated so far is a large number of complexes of polyethers, e.g., polyethylene oxide and polypropylene oxide, with alkali metal salts. However, such polyethers is difficult to obtain high ionic conductivity while maintaining sufficient mechanical strength. Further, the conductivity thereof is considerably influenced by temperature and a sufficient conductivity cannot be obtained at room temperature. For these reasons, investigations have been made on a comb-shaped polymer having polyether side chains, a copolymer of a polyether chain with another kind of monomer, a polysiloxane or polyphosphazene having polyether side chains, a crosslinked polyether, and others.
In ion conductive polymers containing a salt dissolved therein, such as polyether-based polymer electrolytes, both cations and anions move and the transport number of cations at room temperature is usually 0.5 or lower. It has hence been attempted to synthesize a polymer electrolyte type ion conductive polymer which has anionic groups such as --SO.sub.3.sup.- or --COO.sup.- and in which the transport number of lithium ions is unit. However, since lithium ions are strongly attracted by such anionic groups, that polymer has a considerably low ionic conductivity, making it very difficult to use the same in a lithium cell.
It has also been attempted to impregnate a polymer with an electrolyte to produce a gel-state solid electrolyte for use in a lithium cell. Examples of the polymer used in this gel-state solid electrolyte include polyacrylonitrile (J. Electrochem. Soc., 137(1990)1657; and J. Appl. Electrochem., 24(1994)298), polyvinylidene fluoride (Electrochimica Acta, 28(1983)833, 28(1993)591), polyvinyl chloride (J. Electrochem. Soc., 140(1993)L96), polyvinyl sulfone (Electrochimica Acta, 40(1995)2289; and Solid State Ionics, 70/71(1994)20), and polyvinylpyrrolidinone. An attempt has been made to facilitate the infiltration of an electrolyte by using a vinylidene fluoride-hexafluoropropylene copolymer having a reduced crystallinity to thereby improve conductivity (U.S. Pat. No. 5,296,318). It has further been attempted to produce a polymer film by drying a latex of a nitrile rubber, styrene-butadiene rubber, polybutadiene, polyvinylpyrrolidone, or the like and impregnate this film with an electrolyte to produce a lithium ion conductive polymer film (J. Electrochem. Soc., 141(1994)1989; and J. Polym. Sci., A 32(1994)779). In connection with this polymer electrolyte production from a latex, a polymer film designed to combine mechanical strength and ionic conductivity has been proposed which is produced from a mixture of two polymers and has a mixed polymer phase consisting of a polymer phase less impregnable with an electrolyte but having high mechanical strength and a polymer phase easily impregnable with an electrolyte and showing a high ionic conductivity.
Furthermore, reports have been made on a solid electrolyte which comprises a microporous polyolefin film with its pores being filled with a polymer electrolyte so as to improve the mechanical strength and handleability of polymer electrolyte films (J. Electrochem. Soc., 142(1995)683), and on a polymer electrolyte containing inorganic solid electrolyte particles so as to attain an improvement in ionic conductivity, an increase in the transport number of cations, etc. (J. Power Sources, 52(1994)261; and Electrochimica Acta, 40(1995)2101, 40(1995)2197).
Although a large number of various polymer separators and polymer electrolytes have been proposed as described above, there has been no functional film which has essentially overcome the problem concerning the diffusion of lithium ions. Consequently, the performance of batteries containing a nonaqueous electrolyte has been unsatisfactory as compared with batteries containing an aqueous electrolyte.